Construction of membrane-bound replication vacuoles within host cells is a critical disease determinant in a wide swath of pathogens. Within this protected niche, microorganisms are protected from a variety of cellular killing mechanisms as well as cytoplasmic innate immune sensing. Maturation of this compartment is a byproduct of each pathogen parasitizing a specific arm of the host secretory pathway. Legionella pneumophila is one bacterium that uses this strategy, growing in a vacuole within macrophages during pneumonic disease. Construction of the vacuole is promoted by a group of Legionella proteins, which are thought to hijack secretory vesicles emanating from the host cell endoplasmic reticulum (ER) en route to the Golgi. The topography of the host cell, however, is inconsistent with this model, as bacteria often contact host cells at sites distant from vesicle trafficking to the Golgi, arguing that communication with peripheral ER tubules is the first step i replication vacuole construction. The proposed studies will test the model that manipulation of ER tubule function initiates vacuole formation, and that back-up strategies exist to ensure efficient construction of this compartment. Work will focus on bacterial control of host cell reticulon 4 (Rtn 4) isoforms, an evolutionarily conserved set of proteins that physically tubulate ER. In particular, studies will analyze the function of three L. pneumophila Sde family members, proteins that are injected into host cells and target Rtn4. Experiments will be performed to test the model that Sde proteins modify Rtn4 isoforms as a first step in replication vacuole formation and that, in response to this modification, the host protein aggregates into a detergent resistant structure. To pursue this hypothesis, host cells predicted to be unable to support this modification, and Rtn4 mutants altered in the modification site, will be analyzed to determine if they are defective for control of Rtn4 dynamics. In addition, a model will be tested that Sde proteins cause a structural change in a small seed pool of Rtn4 that results in a chain reaction of conformational changes along the length of the ER tubule, reminiscent of prion formation. To account for intracellular growth that occurs in the absence of Rtn4 function, a mutant hunt will be performed to identify L. pneumophila proteins that participate in membrane trafficking pathways in parallel to the Sde family. The consequence of loss of these proteins under conditions in which either the Sde family is missing, or when Rtn4 is forcibly removed from the replication vacuole will be evaluated by determining if the compartment decomposes or routes into an antimicrobial organell in the host cell. In so doing, experiments will be directed toward identifying a weak link in the strategy of manipulating ER tubules to promote intracellular growth, with an eye toward developing antimicrobials that target this process.